1. Field of the Invention
The present invention relates to methods of sterilization, and more particularly, to a method of vapor phase hydrogen peroxide sterilization.
2. Description of the Prior Art
In 1979 Moore et al. disclosed in U.S. Pat. No. 4,169,123 a method of sterilizing with "cold" hydrogen peroxide vapor. Forstrom et al. U.S. Pat. No. 4,169,124 disclosed a similar method wherein the concentration of the hydrogen peroxide vapor is less than 75 mg/L. Bier U.S. Pat. No. 4,642,165 discloses a method of vaporizing multicomponent liquids such as hydrogen peroxide and water, and passing the vapor in successive small increments into a sterilization chamber.
Hydrogen peroxide is rarely, if ever, used in a pure form. It is typically mixed with water. Hydrogen peroxide vapor, like water vapor, is a condensable gas, but is less volatile than water vapor. When injected into a sterilizer with water vapor, the hydrogen peroxide vapor may readily decompose into water and oxygen or condense into liquid. Thus, the concentration of vapor phase hydrogen peroxide within a sterilization chamber will decrease with time as degradation and/or condensation occurs.
It is necessary in order to achieve effective sterilization, to maintain the concentration of the sterilant at effective levels throughout the sterilization phase of a cycle. The Bier patent referenced above discloses that the multicomponent vapor should be introduced in successive increments but it does not address the question of the degradation of the hydrogen peroxide vapor after injection into the sterilization chamber or the problems of exceeding the saturation limit of the vapor mixture.
One problem with indiscriminate successive injections of a hydrogen peroxide and water vapor mixture into the sterilization chamber is that the concentrations of the condensable vapors can exceed the saturation limit of the mixture causing the vapors to condense. The saturation limit generally increases with increasing temperature and varies with the relative concentrations of the different molecular species of the mixture. When the mixture is comprised of hydrogen peroxide and water the problem is compounded because of the degradation of hydrogen peroxide into water and oxygen. The increase in water vapor concentration resulting from such degradation reduces the saturation limit of the mixture and accordingly, reduces the maximum hydrogen peroxide vapor concentration that can be injected into the chamber without causing the vapor mixture to condense.
It is an object of the present invention to provide a vapor phase hydrogen peroxide sterilization method wherein the problems of exceeding the saturation limit of the vapor mixture and the degradation of hydrogen peroxide vapor to water vapor and oxygen are overcome. It is a further object of the present invention to provide an optimum vapor phase hydrogen peroxide sterilization method in order to ensure that an effective level of hydrogen peroxide vapor is present throughout the sterilization stage of the cycle.